A sling is not needed in the sense that its required but one can came in handy. It all depends on what type you are referring to and what you plan to use the rifle for. A traditional sling comes in handy if you are hunting, it can also be use to steady the rifle for more accurate "off hand" shooting. Tactical type 1 or 2 point slings help you carry the rifle in a ready position and free your hands to get to other tools or weapons as needed.
"The whole of the Bill (of Rights) is a declaration of the right of the people at large or considered as individuals.... It establishes some rights of the individual as unalienable and which consequently, no majority has a right to deprive them of." (Albert Gallatin of the New York Historical Society, October 7, 1789)
"A free people ought not only to be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government." - George Washington